A wide variety of configurations of intervertebral implants have been utilized for stabilizing adjacent vertebral elements and facilitating the development of bone union between the vertebral elements. In some configurations, the intervertebral implants are not adjustable by the surgeon during the surgical procedure. Therefore, the surgeon must choose the size that most closely matches the desired height, length and width dimensions, and then make the implant fit. Because these implants are of a predetermined size and shape, the implant site must correspond to the implant configuration. This can require extensive site preparation to complete implantation. Extensive site preparation can compromise the success of the implantation procedure by causing excessive damage to the receiving vertebral elements. In addition, procedures requiring extensive site preparation can result in relatively long surgeries that may increase patient risk.
To address this problem more recent invertebral implants have been designed to expand from a first height to a second height. One such invertebral implant is described in U.S. Pat. No. 6,174,334. This implant includes a pair of shells that when assembled form an implant assembly. Teeth are formed on each shell so that the shells can be unidirectionally spread apart. Each tooth has a ramping surface that is oblique to the line of relative movement of the shells. The ramping surface meets an abutment surface that is perpendicular to the line of relative movement. In other words, each of the teeth formed on the shells has a surface that is 90 degrees relative to the direction in which the shells are spread apart from one another.
It can be understood that as the implant is spread apart, the teeth of the shells ratchet against one another. This ratcheting action causes the lateral walls of the shells to displace laterally. This lateral displacement can cause mechanical stress to a degree at which the shell can experience plastic deformation. If plastic deformation occurs, the two shells of the implant assembly will not interact optimally.
In general, improvement has been sought with respect to such assemblies and systems, generally to provide an expandable implant assembly while maintaining the structural integrity and function of the implant assembly in the event plastic deformation occurs.